We propose to investigate highly phosphorylated and cyclic nucleotides in vivo in both normal and regenerating rat liver, and hepatomas. Impetus for these studies comes from an extensive literature relating both classes of compounds to growth regulation in various systems, and, in particular, implicating cAMP and cCMP in liver regeneration. Added incentive derives from our earlier finding that insulin and glucagon, acting together, are major regulators of liver growth, and from knowledge of their potent influence upon hepatic cyclic nucleotide metabolism. We have begun to examine regenerating liver for changes in labeling profiles of highly phosphorylated and cyclic nucleotides following in vivo administration of Pi32. The two classes of compounds are separated by column chromatography from the same liver extract. Two highly phosphyorylated nucleotides have been discovered. One, compound "A", appears only in livers regenerating for 6 hours or more; the other compound "B", is found in similar amounts in both normal and regenerating livers. Preliminary evidence suggests that both compounds are xanthine derivatives, resembling but not identical to XDP and XTP. After completion of structural analyses of identification we will endeavor to define their metabolic functions. In preliminary experiments we have achieved excellent separation of cAMP, cGMP, cCMP and cUMP when added as unlabeled marker compounds to liver extracts. We plan to survey their relative rates of labeling at intervals after Pi32 injection, and determine the size of the respective hepatic pools, also using a Pi32 labeling procedure. The aim is to extend basic knowledge of the control of liver growth.